Acadia needs to conduct another phase III study of pimavanserin in Parkinson's patients in order to confirm Tuesday's results. Two previous late-stage studies of the drug failed to demonstrate a significant reduction in psychotic episodes.

Treatment with a 40 mg dose of pimavanserin resulted in a 5.79-point reduction in psychotic behavior compared to a 2.73-point reduction in patients treated with a placebo. The 3.06-point difference in the two arms of the phase III study was statistically significant.

Reduction in psychotic behavior focused mostly on hallucinations and delusions and was assessed using a 9-item scale adapted from the Scale for the Assessment of Positive Symptoms (SAPS).

Pimavanserin achieved its antipsychotic effects without worsening motor function in Parkinson's patients, meeting a key secondary endpoint of the trial.

The pimavanserin data are "better than I expected," said Zacks analyst Jason Napodano, in a twitter post reacting to the Acadia study results. The U.S. market opportunity for pimavanserin in Parkinson's disease psychosis is $300-400 million, he adds.

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